The present invention relates to servo systems and motor drive systems and particularly to such systems when used in a video tape recorder (VTR).
Typically, in a videotape recorder the video heads are carried on a rotating member driven by a motor. In order to carry out various operations of the VTR it is necessary to precisely control the rotation of the motor. While Dc motors are particularly suited to such uses, the means for commutating the motor is a problem particularly if high motor efficiency is desired so that heat generation is minimized near the motor and associated critically dimensioned mechanical elements. Prior art DC motor commutation has included the use of Hall devices which reduce the motor efficiency and various optically commutated approaches which sequentially energize the motor stator windings. However, these prior art approaches do not optimize motor efficiency nor are they easily adapted to applications where control voltages are applied for causing small changes in the motor rotation as is required in synchronizing a VTR.
There are many applications, including synchronizing-generator locking in television studios and the synchronizing of VTR's to external reference timing signals wherein two control variable signals, such as a horizontal line rate synchronizing signal and a vertical (frame or field) rate synchronizing signal (each in frequency and phase lock with each other) are both to be phase locked to two external reference horizontal rate and vertical rate signals. A prior art approach in the generator locking art is to lock the local horizontal to the reference horizontal in a phase locked loop, then to compare the reference vertical and local vertical phase and "slip" the local vertical by increments of horizontal lines until vertical phase lock is achieved. For example, see U.S. Pat. No. 2,720,555 to I. A. Krause. While this approach is acceptable in generator locking because the local vertical is related to the local horizontal in a count down divider chain, it is not applicable to the VTR situation where the local vertical and local horizontal are inherently tied to each other. That is, in order to rephase the local vertical, the local horizontal must also shfit unlike the generator locking case where the local vertical can be moved independently of the local horizontal.
Prior art VTR approaches to this phase locking problem have typically sought to achieve vertical phase lock by developing an analog error signal representing the vertical phase error and using this error signal to provide an open loop steering of the capstan servo loop, hence moving the horizontal and consequently, rephasing the vertical. Such approaches tend to be rather abrupt, imprecise and uncontrolled because they do not retain a closed loop situation during the correction step or steps.
An ancillary problem in VTR's is the precise control of the videotape in shuttle and stop modes. In manual operation and particularly in automatic or semi-automatic search modes where it is desired to rapidly find and park at a particular address on the tape the prior art VTR's have provided imprecise open loop shuttling and stopping.
A further problem in high quality VTR's is the introduction of tape tension correction in the video head area. Due to contraction or expansion of the videotape between recording and playback, which often may be months or years under widely variant temperature and humidity, it it necessary to provide a mechanical stretching across the head area so that the original tape dimensions with respect to the recorded signals are duplicated as closely as possible. The prior art has made this correction by driving a flywheel type oscillator with the off-tape horizontal sync signals and generating an error signal by comparing the instantaneous off-tape horizontal signals with the oscillator output once at the head switch time. This single sample approach has the disadvantage of permitting common mode DC errors to decrease the accuracy of the correction signal. Moreover, the prior art error signals have typically been used to control electromagnetic means on one side of the tape head area acting against the capstan on the other side of the tape head area, thus requiring a relatively high friction tape path.